The present invention relates to a transmission line switching control method and apparatus comprising a plurality of transmission lines, for example, a current transmission line and a spare transmission line for automatically switching the current transmission line to the spare transmission line when a fault occurs in the current transmission line. More specifically, the invention relates to a transmission line switching control method and apparatus for controlling switching of the transmission line when a fault occurs in the transmission line by receiving fault information indicating the fault and switching control data sent by a switching communication channel. The conventional transmission line has a redundant structure, for example, so as to prevent line disconnection due to an occurrence of a fault in the transmitter. The redundant structure is a transmission line structure that a spare transmission line is prepared besides the current transmission line which is now in use. For switching the transmission line having such a redundant structure, a switching control channel is mounted in the transmission line, and both stations (the self transmitter and an opposite transmitter opposite to the above transmitter) communicate the fault status mutually (transmit or receive data), and the transmission line is switched by the above communication. As an example thereof, a transmission system switching control unit is disclosed in Japanese Patent Laid-Open No. 60-180340. The transmission system switching control unit comprises a transmitter status reading circuit, a switching unit status reading circuit, a load status reading circuit, a storage circuit, a central processing unit (CPU), a transmitter control circuit, and a switching unit control circuit, and when a fault occurs in the transmitter, decides whether the fault occurs in the transmitter now in use or in the spare transmitter or the status which is read by each circuit by the central processing unit, and switches the current transmitter to the spare transmitter when the fault occurs in the current transmitter.
However, according to such a transmission system switching control unit, fault occurrence information and information read by each circuit are all read and processed by the central processing unit, in other words, there are many conditions to be decided such as receiving switching control data by the processing unit, etc. Therefore, it requires a lot of time so as to complete the execution of switching. Particularly when a plurality of transmission lines are processed, each transmission line is processed by the central processing unit (CPU) on a time series basis. Therefore, the operation time required for switching control by the CPU and the time required from an occurrence of a fault to completion of switching increase in proportion to the number of transmission lines. Furthermore, as the number of transmission lines to be installed increases, the operation time of the control CPU of the equipment required for switching control increases and it causes difficulties to another control. In proportion to an increase in the operation time of the control CPU of the equipment, the time required for recovery of the fault status of the transmission line also increases.